The present invention relates to an optical recording medium such as a phase-change optical recording medium.
In recent years, attention has been paid to an optical recording medium capable of carrying out high density recording, and furthermore, erasing and rewriting the recorded information. A rewritable phase-change optical recording medium of serves to irradiate a laser beam to change the crystal state of a recording layer, thereby carrying out recording and to detect a change in the reflectance of the recording layer with such a change in the state, thereby carrying out reproduction. Since the optical system of a driving device is simpler than that of a magneto-optical recording medium, attention has been paid to the phase-change optical recording medium.
Since a difference in a reflectance between a crystalline state and an amorphous state is great and the stability of the amorphous state is comparatively high, a chalcogenide based material such as a Gexe2x80x94Sbxe2x80x94Te based material is often used for the phase-change recording layer.
When information is to be recorded in the phase-change optical recording medium, a laser beam having such a high power (recording power) that the temperature of the recording layer is raised to a melting point or more is irradiated. In a portion in which the recording power is applied, the recording layer is molten and is then quenched to form an amorphous recording mark. On the other hand, when the recording mark is to be erased, a laser beam having such a comparatively low power (erasing power) as to raise the recording layer to have a temperature which is equal to or higher than a crystallization temperature thereof and is less than a melting point is irradiated. The recording mark having the erasing power applied thereto is heated to the crystallization temperature or more and is then cooled slowly so that it returns to be a crystalline substance. In the phase-change optical recording medium, accordingly, the intensity of a single laser beam is modulated so that overwrite can be carried out.
In order to realize an increase in a recording density, a recording and reproducing wavelength has been shortened and the numerical aperture of the objective lens of a recording and reproducing optical system has been increased. A spot size on the surface of the recording layer of a laser beam for recording is represented by xcex/NA, wherein a laser wavelength is represented by xcex and a numerical aperture is represented by NA.
The reproducing light is originally a laser beam having a lower power which does not influence the crystal state of a recording mark. When the reproduction is repetitively carried out, the recrystallization of the recording mark is generated. As a result, a jitter is increased. According to the study of the inventors, in the case in which a laser beam having a short wavelength is irradiated by an optical system including an objective lens having a great numerical aperture for high density recording, that is, the xcex/NA is small, an energy density in a laser beam spot is increased. Consequently, it has been found that the jitter is increased, that is, a reproducing durability is greatly deteriorated even if the number of repetitive reproductions is comparatively small.
When a laser beam for recording is irradiated, the temperature of an adjacent track to a recording object track is also raised. For this reason, an amorphous recording mark present on the adjacent track to the recording object track is recrystallized in some cases. Such a phenomenon will be referred to as a cross erase in this specification. When the cross erase is generated, a carrier to noise ratio (C/N) is reduced. If a recording track pitch for a laser beam spot size is smaller, the cross erase is generated more easily. According to the study of the inventors, moreover, it has been found that the cross erase is generated more easily with a higher energy density in a beam spot even if the ratio of the beam spot size to the recording track pitch is equal.
It is an object of the invention to provide an optical recording medium having an excellent reproducing durability and capable of suppressing a cross erase, and furthermore, to provide an optical recording medium having a small jitter during high density recording.
Such an object is achieved by the invention according to the following (1) to (5).
(1) An optical recording medium wherein a recording layer, an absorbent layer and a radiating layer are provided in this order as seen from a recording/reproducing light incidence side and recording/reproduction is carried out on a condition that an extinction coefficient k of the absorbent layer is k greater than 0 in a wavelength of recording/reproducing light, and
a relationship between a wavelength xcex of the recording/reproducing light and a numerical aperture NA of an irradiating optical system of the recording/reproducing light is xcex/NAxe2x89xa6680 nm.
(2) The optical recording medium according to the (1), wherein a recording track pitch PT is set to PTxe2x89xa60.35 xcexcm.
(3) The optical recording medium according to the (1) or (2), wherein an absorbent layer is provided on this side of the recording layer as seen from a recording/reproducing light incidence side and an extinction coefficient k of the absorbent layer is k greater than 0 in a wavelength of recording/reproducing light, and
a signal including a minimum mark having a mark length Ms of MSxe2x89xa60.17 xcexcm is recorded.
(4) An optical recording medium wherein an absorbent layer and a recording layer are provided in this order as seen from a recording/reproducing light incidence side and an extinction coefficient k of the absorbent layer is k greater than 0 in a wavelength of recording/reproducing light, and
a signal including a minimum mark having a mark length Ms of MSxe2x89xa60.17 xcexcm is recorded.
(5) The optical recording medium according to any one of the (1) to (4), wherein the extinction coefficient k of the absorbent layer is preferably within the range of 0.06 to 1.0.